1. Field of the Invention
The present invention relates to an electrical heating catalyzer, in particular, to an electrical heating catalyzer capable of uniformly heating base material thereof.
2. Description of the Related Art
Conventionally, a catalyzer is used as an exhaust emission control device for purifying exhaust from an engine. The catalyzer is configured by a catalyst, such as Pt, Pd, or Rh, being supported by a ceramic honeycomb body (base material), and is capable of achieving desired exhaust emission control performance by being held at a predetermined activation temperature (such as about 400° C.).
For example, as a catalyzer disclosed in JP-A-H3-295184, an electrical heating catalyzer (EHC) is proposed in which a pair of electrodes are provided in a honeycomb body, and the honeycomb body is heated by voltage being applied between the electrodes. More specifically, the honeycomb body is formed into a circular columnar shape by an electrically conductive ceramic, such as silicon carbide, and has numerous through-holes (cells) extending in a direction in which exhaust passes. The pair of electrodes in the form of thin films are provided on the outer peripheral surface of the honeycomb body in two positions that, for example, differ by 180 degrees with an axial center of the honeycomb body as reference. As a result of voltage being applied to the pair of electrodes, the honeycomb body is heated.
Early activation of the catalyzer is required to be achieved to enhance exhaust emission control performance in an engine, and therefore, electrical energy introduced to the honeycomb body during heating thereof has been increasing in recent years. For example, in some instances, a large-capacity battery is mounted in a hybrid car, enabling high power to be introduced to the honeycomb body. The honeycomb body may be able to be heated to about 500° C. in just 20 to 30 seconds.
On the other hand, in the electrical heating catalyzer, when energization is performed by voltage being applied to the pair of electrodes provided in the honeycomb body, variation is considered to occur in the current flowing through each section of the honeycomb body as a result of placement aspect of the electrodes, individual differences and aging of the honeycomb body, and the like. For example, the size of the current differs between a center section and an end section of the electrode, and energy (current) is concentrated in the end section of the electrode. In this instance, because the honeycomb body is not uniformly heated, thermal stress is concentrated locally, thus possibly leading to problems such as damage to the honeycomb body. Such problems are considered especially noticeable when the introduced energy is increased to achieve early activation (early temperature rise) of the catalyzer, as described above.